A supersonic combustion ramjet engine, referred to as a `scramjet`, is a potential propulsion system for flight vehicles traveling at hypersonic speeds. Hypersonic typically refers to speeds greater than Mach 4. A typical scramjet is comprised of a passage, some form of fuel injector, and an exhaust nozzle. The motion of the aircraft directs the fluid into the passage where it is compressed. Fuel is injected into the compressed fluid and burned. The products of combustion are then ejected through the exhaust nozzle to provide forward thrust to the aircraft. An advantage of this means of propulsion is its simplicity.
Various types of fuel injectors have been proposed. In the first type, fuel is injected normal to the direction of flow, The normal injection improves mixing and thereby fuel burning efficiency. Drawbacks to this type of injection include transmission of a pressure rise upstream of the combustion that may interfere with the flow through the combustion region and reduced mixing effectiveness at hypersonic speeds.
Another type of fuel injection comprises injecting fuel in the direction of flow through the combustor. The advantage to this type of fuel injection is that the momentum of the fuel injected into the fluid stream contributes to the thrust created by the engine. This feature is especially significant at hypersonic speeds. At speeds less than hypersonic, however, axial injection may lead to poor mixing.
U.S. Pat. No. 5,253,474, issued to Correa et al and titled "Apparatus for Supersonic Combustion in a Restricted Length", discloses a combustor having axial fuel injection and normal fuel injection downstream of a backward facing step. According to Correa, the backward facing step avoids coupling between the inlet flow and the combustion process. Both the axial and normal fuel injection occur simultaneously in Correa throughout the speed range. This results in axial injection at speeds less than hypersonic and normal injection at hypersonic speeds, neither of which is ideal.
Heat loads on the structure is another issue related to the fuel injectors. The combination of the adjacent combustion process and the supersonic flow results in large heat loads on the fuel injectors. Cooling is required to prevent heat damage from occurring. Typically, the cooling comes from flowing the fuel through the injector before ejecting it into the flow path.
An example of such a cooling scheme is disclosed in U.S. Pat. No. 4,903,480, issued to Lee et al and rifled "Hypersonic scramjet Engine Fuel Injector". The fuel injector disclosed in Lee is a hollow wedge shaped housing having an interior serpentine fuel passageway. Fuel traveling through the passageway cools the injector. In this way cooling flow and fuel flow are linked, i.e. high fuel flow results in maximum cooling flow and low fuel flow results in lower cooling flow. In practice, however, heat loads and fuel flow may not be directly related. This would result in too little cooling under some conditions and excessive cooling flow, and its associated pressure drops within the fuel flow, at other conditions.
The above art notwithstanding, scientists and engineers under the direction of Applicants' Assignee are working to develop fuel injectors and fuel injection systems for scramjet engines capable of providing thrust efficiently at both hypersonic and less than hypersonic speeds.